The dentate gyrus is one of only two brain regions that continue to produce large numbers of new neurons during adulthood. The goal of our research is to understand the function of adult neurogenesis by studying the regulation of granule cell birth, the properties of the new neurons, and the behavioral consequences of altering neurogenesis. One focus of our work is exploring the basic developmental processes that continue in the rat dentate gyrus throughout adulthood. Many factors, such as hormones, neurotransmitters, and environmental manipulations regulate several aspects of granule cell development. We are currently using in vivo injections of the S-phase marker bromodeoxyuridine to examine how these factors converge to control the opposing processes of cell birth and cell death. Understanding the dynamics of the granule cell population may provide clues to the function of the new neurons, e.g., whether they replace old granule cells or increase the size of the population. This work might also suggest ways to encourage neurogenesis in other brain regions. Another aspect of our work involves exploring the effects of corticosteroids on the hippocampus. Psychosocial stress and corticosteroids, hormones released from the adrenal in response to stress, inhibit neurogenesis in the developing and adult dentate gyrus. We have found that corticosteroids are responsible for the inhibition of neurogenesis observed in very old rats; removing corticosteroids from aged rats restores the rate of neurogenesis to that seen in young adults. Work is currently underway to elucidate that relationship between corticosteroids, neurogenesis, and hippocampus-dependent memory. This work is likely to have implications for benign senescent memory loss in humans, as well other conditions associated with high corticosteroid levels and structural changes in the hippocampus, including chronic stress, major depression, and therapeutic use of corticosteroid medications.